In the article "Oxidation-Reduction-Potential-A Tool for Monitoring, Control, and Optimization of Biological Nutrient Removal Systems" in WAT. SCI. TECH. vol. 17 Paris 1985, p. 259-281, it is shown that the redox potential is an effective index for the end of the denitrification phase. An inflection point in the time curve of the redox potential, which indicates the end of the denitrification phase, is used for the control. A problem with the use of this value consists in the reliable identification of the inflection point, called the redox inflection point below. Furthermore, the identification is complicated by the fact that the absolute value of the redox potential in the area of the inflection point varies enormously in the case of different framework conditions. In the above-mentioned article, values of 100 to 350 mV are indicated for the absolute value of the redox potential in the area of the inflection point. The absolute value of the redox potential thus cannot be reasonably used for facilitating the identification of the inflection point, although this also is proposed in EP 396 057 B1. The absolute value of the redox potential is used there in addition to the identification of the inflection point. The end of the denitrification phase is assumed when an inflection point has been detected and, additionally, the absolute value of the redox potential is within a predetermined interval. However, from the article indicated above it is obvious that the absolute value of the redox potential fluctuates considerably in relation to the existing framework conditions, because of which this parameter just does not increase the reliability of the identification. A further criterion is seen in EP 396 057 B1 in the absence of oxygen in the waste water. Thus, in addition to the redox inflection point, the absence of oxygen in the waste water also is used as a switch criterion. However, this parameter is also not a reliable index for the end of the denitrification phase, because as a rule the absence of oxygen in the denitrification phase can be relatively early. Thus, switching off too early is excluded by means of this parameter only at the beginning of the denitrification.